Question

A student is deciding which chemicals are acids by measuring their pH. He sees that a solution of CO2 has an acidic pH. He decides that he must have contaminated his sample. He thinks that CO2 could not create hydrogen ions in the solution since it contains no hydrogen atoms. Explain what is incorrect about the student's reasoning.

Answer 1

He has forgotten that water itself has hydrogen atoms, and can produce hydrogen ions, and that therefore anything that can chemically react with water, whether it has hydrogen atoms itself or not, can change the pH by producing hydrogen ions with its reaction with water.

In this case, CO2 reacts with water to produce carbonic acid, H2CO3, as well as the bicarbonate anion HCO3-, hydronium ion H3O+, and a little bit of carbonate anion CO3-2. There's the source of the H3O+.

Answer 2

Thanks! That was a really well written explanation. I understand it much better!